This paper presents an information-theoretic analysis of security for data hiding methods based on spread
spectrum. The security is quantified by means of the mutual information between the observed watermarked
signals and the secret carrier (a.k.a. spreading vector) that conveys the watermark, a measure that can be used
to bound the number of observations needed to estimate the carrier up to a certain accuracy. The main results of
this paper permit to establish fundamental security limits for this kind of methods and to draw conclusions about
the tradeoffs between robustness and security. Specifically, the impact of the dimensionality of the embedding
function, the host rejection, and the embedding distortion in the security level is investigated, and in some cases
In this paper, security of lattice-quantization data hiding is considered under a cryptanalytic point of view. Security in this family of methods is implemented by means of a pseudorandom dither signal which randomizes the codebook, preventing unauthorized embedding and/or decoding. However, the theoretical analysis shows that the observation of several watermarked signals can provide sufficient information for an attacker willing to estimate the dither signal, quantifying information leakages in different scenarios. The practical algorithms proposed in this paper show that such information leakage may be successfully exploited with manageable complexity, providing accurate estimates of the dither using a small number of observations. The aim of this work is to highlight the security weaknesses of lattice data hiding schemes whose security relies only on secret dithering.
Until now, the sensitivity attack was considered as a serious threat to the robustness and security of spread spectrum-based schemes, since it provides a practical method of removing watermarks with minimum attacking distortion. Nevertheless, it had not been used to tamper other watermarking algorithms, as those which use side-information. Furthermore the sensitivity attack has never been used to obtain falsely watermarked contents, also known as forgeries. In this paper a new version of the sensitivity attack based on a general formulation is proposed; this method does not require any knowledge about the detection function nor any other system parameter, but just the binary output of the detector, thus being suitable for attacking most known watermarking methods, both for tampering watermarked signals and obtaining forgeries. The soundness of this new approach is tested by empirical results.
The performance of quantization-based data hiding methods is commonly analyzed by assuming a flat probability density function for the host signal, i.e. uniform inside each quantization cell and
with its variance large enough to assuming that all the centroids occur with equal probability. This paper comes to fill a gap in watermarking theory, analyzing the exact performance of the Scalar Costa Scheme (SCS) facing additive Gaussian attacks when the former approximation is not valid, thus taking into account the host statistics. The accomplished analysis reveals that the true performance of such a scheme for an optimal selection of its parameters and low watermark to noise ratios (WNR) is never worse than that of classical spread-spectrum-based methods, in terms of achievable rate and probability of error, as it was thought so far. The reduction of SCS to a two-centroid problem allows the derivation of theoretical expressions which characterize its behavior for small WNR's, showing interesting connections with spread-spectrum (SS) and the Improved Spread Spectrum (ISS) method. Furthermore, we show that, in contrast to the results reported until now, the use of pseudorandom dithering in SCS-based schemes can have a negative impact in performance. Performance losses are also reported for the case in which a modulo reduction is undertaken prior to decoding. The usefulness of these results is shown in the computation of the exact performance in projected domains.
In this paper, a novel method for detection in quantization-based
watermarking is introduced. This method basically works by quantizing a projection of the host signal onto a subspace of smaller dimensionality. A theoretical performance analysis under
AWGN and fixed gain attacks is carried out, showing great improvements over traditional spread-spectrum-based methods operating under the same conditions of embedding distortion and attacking noise. A security analysis for oracle-like attacks is also accomplished, proposing a sensitivity attack suited to quantization-based methods for the first time in the literature, and showing a trade-off between security level and performance; anyway, this new method offers significant improvements in security, once again, over spread-spectrum-based methods facing the same kind of attacks.